In Freudian psychoanalytic theory, defense mechanisms are unconscious psychological strategies brought into play by various entities to cope with reality and to maintain self-image. Healthy persons normally use different defenses throughout life. An ego defense mechanism becomes pathological only when its persistent use leads to maladaptive behavior such that the physical and/or mental health of the individual is adversely affected. The purpose of ego defense mechanisms is to protect the mind/self/ego from anxiety, social sanctions or to provide a refuge from a situation with which one cannot currently cope.(http://en.wikipedia.org/wiki/Defence_mechanism).
Because of anxiety provoking demands created by the id, superego and reality, the ego has developed a number of defense mechanisms to cope with anxiety. Although we may knowingly use these mechanisms, in many cases these defenses work unconsciously to distort reality.(http://psychology.about.com/od/theoriesofpersonality/ss/defensemech_2.htm)
In english, the term "defense mechanism" is often thought to refer to a definitive singular term for personality traits which arise due to loss or traumatic experiences, but more accurately refers to several types of reactions.
Probably you would wonder why I'm talkin' about this stuff? Well, I'm not a psychology major but I tend to observe people's behavior, and I don't do it on purpose. I'm not that talkative kind of person, I just sit and observe. And just last night, my sister has been acting kinda odd. I mean, I know how she acts when she's in a good mood, she's always hyper and noisy, and if she's not on the mood she just stays in the room alone and read a book, or do anything. But last night, idrk but I sense she's not really happy. Then just today, this morning, I read on her planner that, she just broke up w/ her bf 'cause of some misunderstanding. Excuse me, I don't know why she wrote it in her planner, it was called a -planner- because you write their your daily schedules, not the-daily-happenings-in-our-life, and it's not that I rummage on her personal properties, it was just an accident. Back to the topic, well, I guess those information she wrote in her planner already answered the questions in my head. This afternoon, I was in my room, then suddenly she entered with watery eyes and told me to get out, and I did, even if she won't tell me, I could feel what she felt that moment. And just tonight, I've been watching how she behaves, and I observed she's behaving like everything's normal, I don't know if they got back together, but if yes, I'm happy for her, but if not, well, she's just acting out everything, and the DEFENSE MECHANISM enters here. To be more specific, there are different types and levels of defense mechanism, but here, I think she's just acting out everything so that: 1- my parents wouldn't ask her those annoying questions about what happened. 2- she doesn't want us to think that she's desperate about the break up. 3- she want us, her younger sibling, to see that she's a strong person. 4- she wants everyone to think that it's not a big deal. Reasons like that, she is acting out to protect herself and to outrun the problem. She thinks that if she will pretend that everything is A-OKAY, everything will just follow, as long as her self-image is not affected by the problem. She does not reflect on her internal feelings which is bad because if she does not face the reality, she would have hard time coping with her problem. While all defense mechanisms can be unhealthy, they can also be adaptive and allow us to function normally. The greatest problems arise when defense mechanisms are overused in order to avoid dealing with problems.http://psychology.about.com/od/theoriesofpersonality/ss/defensemech_2.htm
Sometimes these defense mechanisms are not really the answer to our problems, we just have to accept and face the fact and reality that -"life is not just all about happiness and luxury. We have to face that there are hard and bad times in it, and if we encounter and overcome those, it builds us to become a strong person".